1. Field of the Invention
This invention relates to a semiconductor device and, more particularly, to a semiconductor laser device capable of emitting blue light.
2. Prior Art
II-VI group compounds are attracting attention as such compounds seem particularly promising for the generation of blue laser.
Papers Nos. 1 and 2 listed below describe some such materials capable of emitting blue light.
Paper No. 1: H. Jeon et al., Appl. Phys. Lett., 59 (1991), 3619. PA1 Paper No. 2: W. Xie et al., Col. of Papers for 1992 spr. 28a, Japan Institute of Applied Physics PA1 E.sub.Afp : quasi-Fermi level for the hole in the active layer under lasing condition, PA1 E.sub.Av : energy level for the valence electron band edge of the active layer under lasing condition, PA1 E.sub.CLfp : quasi-Fermi level for the hole in the p cladding layer under lasing condition, and PA1 E.sub.CLV : energy level for the valence band edge of the p cladding layer under lasing condition. PA1 Paper No. 3: K. Iga et al., Electron Lett., 22, 1008 (1986) PA1 Paper No. 4: K. Iga et al., Conference on Laser and Electro-Optics, California, Tech. Digest 12 (1992) 2. PA1 Paper No. 5: F. L. Schuermeyer et al., Appl. Phys. Lett., 55, 1877 (1989) PA1 Paper No. 6: M. Irikawa et al., Jpn. J. Appl. Phys. 31 (1992) L1351
Each of the semiconductor laser devices described in these papers comprises, as typically illustrated in FIG. 4 of the accompanying drawings, an n+-GaAs buffer layer 1a, an n-ZnS.sub.0.054 Se.sub.0.945 cladding layer 2, a 0.5 .mu.m thick n-ZnSe (Cl doping rate: 5.times.10.sup.17 cm.sup.-3) optical confinement layer 3, an active layer 4 having a multiquantum well structure constituted by a Cd.sub.0.2 Zn.sub.0.8 Se well layer 4b and a ZnSe barrier layer 4a, a 0.5 .mu.m thick p-ZnSe (N doping rate: 4.times.10.sup.17 cm.sup.-3) optical confinement layer 5, a 1 .mu.m thick p-ZnS.sub.0.054 Se.sub.0.945 cladding layer 6 and a 1,000 .ANG. thick p.sup.+ -ZnSe cap layer 7, sequentially laid on a n.sup.+ -GaAs substrate 1 to form a multilayer structure.
Semiconductor laser devices involving a II-VI group compound are, however, not without problems. One of the problems is the difficulty of p- and n-type doping and the other is that they often tend to show poor performances mainly because they have a rather low heterobarrier.
Fortunately, the first problem has been substantially resolved as p-type doping operations can be carried out to a degree of 10.sup.18 cm.sup.-3, if plasma is used for nitrogen doping.
The second problem, on the other hand, is intrinsically a difficult problem, because there is no combination of materials having a large heterobarrier (a large difference in the bandgap) and emitting blue light and at the same time having a lattice-matching feature.
In other words, if a semiconductor laser device is prepared, using such a compound, either of the active layer or the carrier confining cladding layer of the device needs to be a strained superlattice layer.
The amount of strain allowed to a strained layer, however, is limited because of the restriction posed on such a layer by the relationship between the critical layer thickness and the amount of strain. Hence, inevitably the difference in the band gap between the active layer and the cladding layer of a semiconductor laser device of the type under consideration cannot be made satisfactorily large.
The height of the heterobarrier in a double heterojunction structure at the time of laser oscillation will be briefly described below.
The heterobarrier .DELTA.E.sub.c at the conduction band side of the active and p cladding layers is generally expressed by formula (1) below, if the active layer is p-doped. EQU .DELTA.E.sub.c =.DELTA.E.sub.g -.vertline.(E.sub.Afp -E.sub.Av)-(E.sub.CLfp -E.sub.CLV .vertline. (1)
where .DELTA.Eg: difference in the band gap,
The value within .vertline. .vertline. normally becomes small when the p cladding layer is sufficiently doped. Then, a small value for .DELTA.E.sub.g simply signifies a small heterobarrier in conduction band.
Incidentally, the value of the second term within .vertline. .vertline. increases to further reduce the value of .DELTA.E.sub.c, if the p cladding layer is insufficiently doped.
By referring to the Paper No. 1 listed above, the value for .DELTA.E.sub.g between ZeSe and Cd.sub.0.2 Zn.sub.0.8 Se is reported to be 0.26 eV for a conventional semiconductor laser device as illustrated in FIG. 4 of the accompanying drawings.
This value is about 40% smaller than .DELTA.E.sub.g =:0.4 eV of a GaInAsP/InP system for 1.3 .mu.m lasers. If the carrier concentration injected into the active layer is assumed to be 3 to 6.times.10.sup.18 cm.sup.-3, the second term of formula (1) will be 50 to 80 meV.
On the other hand, the lowest quantum energy level of a quantum well is as high as about 50 meV from the conduction band edge and the quasi-Fermi level for electrons in the active layer becomes higher than this quantum energy level by 40 to 70 meV. Hence, the heterobarrier height will be about 60 to 120 meV as measured from the quasi-Fermi level of the active layer.
Thus, since .DELTA.E.sub.c of a conventional semiconductor laser device having a configuration as illustrated in FIG. 4 does not have a sufficiently large .DELTA.E.sub.c and, because of this, carriers or electrons in particular easily overflow to the p cladding layer side to generate leakage currents, which by turn prevents CW oscillation at a high temperature range above room temperature, deteriorating the performance of the semiconductor device for emitting blue light.
In view of the above identified technological problems, it is therefore an object of the present invention to provide a semiconductor laser device capable of emitting blue light and having a sufficiently large .DELTA.Ec for CW oscillation in a high temperature range above room temperature and other desired performances.